Armchair Janus WSSe Nanotube Designed with Selenium Vacancy as a Promising Photocatalyst for CO2 Reduction

Photocatalytic conversion of carbon dioxide into chemical fuels offers a promising way to not only settle growing environmental problems but also provide a renewable energy source. In this study, through first-principles calculation, we found that the Se vacancy introduction can lead to the transition of physical-to-chemical CO2 adsorption on Janus WSSe nanotube. Se vacancies work at the adsorption site, which significantly improves the amount of transferred electrons at the interface, resulting in the enhanced electron orbital hybridization between adsorbents and substrates, and promising the high activity and selectivity for carbon dioxide reduction reaction (CO2RR). Under the condition of illumination, due to the adequate driving forces of photoexcited holes and electrons, oxygen generation reaction (OER) and CO2RR can occur spontaneously on the S and Se sides of the defective WSSe nanotube, respectively. The CO2 could be reduced into CH4, meanwhile, the O2 is produced by the water oxidation, which also provides the hydrogen and electron source for the CO2RR. Our finding reveals a candidate photocatalyst for obtaining efficient photocatalytic CO2 conversion.


Introduction
For the last few years, given the limitation of fossil fuel reserves and the growth of atmospheric CO 2 levels, an urgent need has existed to create a sustainable option for converting unwanted CO 2 into useful products in the form of chemicals and fuels [1][2][3], which will not only solve the greenhouse effect, melting glaciers, and other environmental problems caused by carbon dioxide, but also alleviate the current energy crisis [4]. The conversion of carbon dioxide could be operated through a variety of pathways, including biochemical [5], electrochemical [6,7], photochemical [8,9], and thermochemical [10] reactions. As sunlight is a theoretically unlimited power source, solar-powered CO 2 reduction can be perceived as the best option among these promising approaches [11,12]. Until now, photocatalytic CO 2 RR has attracted great attentions and achieved many results [13][14][15][16]. Photocatalysis is widely believed to have three primary key steps, i.e., sunlight harvesting by the semiconductor (hν > Eg), photo-generated carrier separation and transport, and reactions on the surface [17][18][19][20]. While many solar active catalysts for CO 2 photoreduction have been reported, they mostly suffer from instability, poor energy conversion rates, non-controllable selectivity, and failure to fully inhibit competing hydrogen evolution reactions (HER) in existence with water [21,22]. Consequently, it remains a great priority 2. Results and Discussion 2.1. The CO 2 Adsorption on Pristine Janus WSSe Nanotube Janus WSSe nanotubes are constructed by scrolling Janus WSSe monolayers, whereby the W layer is interposed between the Se and S layers. Our previous work reported that the strain energy for the formation (0.10 eV/atom) of the Janus WSSe nanotubes with a structure of Se layer on the outside and S layer on the inside is lower than the one (0.23 eV/atom) with a contrary structure, indicating relatively more stability [27]. Herein, we chose the (12,12) armchair Janus WSSe nanotube as the substrate in the adsorption system. As shown in Figure S1, the diameter is 21.86 Å and the height of Se-S is 3.22 Å. The W-S and W-Se bond lengths are 2.38 and 2.60 Å, respectively, separately a little shorter and larger than the corresponding ones (2.41 and 2.52 Å) in the planar structure [28]. In this study, we only considered the CO 2 adsorption on the outer side (Se side) of the nanotube, and the case of the adsorption on the inner side is neglected because the CO 2 gas molecules are difficult to pass through the nanotube walls to arrive on the inner side (the barrier is up to 28.33 eV, see Figure S2). We put a CO 2 gas molecule on the Se side of the nanotube to build the adsorption system and completely relax it. As shown in Figure 1, there are four adsorption sites taken into consideration, namely center (above center of the hexagon), bond (above W-Se bond), and W/Se (above W/Se atom). According to Equation (1), we obtained Eads values of various adsorption sites, which were used to explore the most stable adsorption configuration. As shown in Figure 1b, we found that the Eads arrived the smallest (−0.19 eV) when the adsorbed CO2 gas molecule was located at the center site, which was the most stable adsorption configuration. The small absolute value of Eads of this adsorption configuration revealed that the adsorption is physical adsorption (usually, | ads | ≤ 1 eV [29][30][31][32]).
We studied the mechanism of the CO2 physisorption on the pristine Janus WSSe nanotube in detail based on the adsorption distance and Bader charge results. The CO2 gas molecule kept the linear morphology after adsorption (see Figure 1c), and the distance from the C atom of the CO2 gas molecule to its nearest Se atom of the pristine Janus WSSe nanotube is as high as 3.54 Å , which greatly exceeds the Se-C bond length (2.29 Å ). In addition, the amount of transfer electron, moving from the pristine Janus WSSe nanotube to the CO2 molecule, is only 0.02 e, indicating the weak interaction between the substrate and the CO2 molecule.
At the same time, we also calculated the DOS values of the adsorption configurations. As can be seen from Figure 2a-c, the projected DOS of the WSSe nanotube has a negligible change compared with those of the corresponding pristine WSSe nanotube, indicating that the electronic properties of the WSSe nanotube remain. However, there is a significant difference of the DOS between the adsorbed gas molecule and the pristine gas molecule, which is due to charge rearrangement after adsorption; that is, the O atoms gain electrons, while the C atom loses electrons (as listed in Table S1). The little orbital hybridization between the WSSe nanotube and CO2, mainly composed of the Se p and CO2 O p orbitals, is consistent with the tiny interfacial electron transfer, demonstrating that the interaction between the WSSe nanotube and molecules is weak. According to the above analysis, it can be determined that the adsorption of CO2 by the pristine WSSe nanotube is physisorption. According to Equation (1), we obtained E ads values of various adsorption sites, which were used to explore the most stable adsorption configuration. As shown in Figure 1b, we found that the E ads arrived the smallest (−0.19 eV) when the adsorbed CO 2 gas molecule was located at the center site, which was the most stable adsorption configuration. The small absolute value of E ads of this adsorption configuration revealed that the adsorption is physical adsorption (usually, |E ads | ≤ 1 eV [29][30][31][32]).
We studied the mechanism of the CO 2 physisorption on the pristine Janus WSSe nanotube in detail based on the adsorption distance and Bader charge results. The CO 2 gas molecule kept the linear morphology after adsorption (see Figure 1c), and the distance from the C atom of the CO 2 gas molecule to its nearest Se atom of the pristine Janus WSSe nanotube is as high as 3.54 Å, which greatly exceeds the Se-C bond length (2.29 Å). In addition, the amount of transfer electron, moving from the pristine Janus WSSe nanotube to the CO 2 molecule, is only 0.02 e, indicating the weak interaction between the substrate and the CO 2 molecule.
At the same time, we also calculated the DOS values of the adsorption configurations. As can be seen from Figure 2a-c, the projected DOS of the WSSe nanotube has a negligible change compared with those of the corresponding pristine WSSe nanotube, indicating that the electronic properties of the WSSe nanotube remain. However, there is a significant difference of the DOS between the adsorbed gas molecule and the pristine gas molecule, which is due to charge rearrangement after adsorption; that is, the O atoms gain electrons, while the C atom loses electrons (as listed in Table S1). The little orbital hybridization between the WSSe nanotube and CO 2 , mainly composed of the Se p and CO 2 O p orbitals, is consistent with the tiny interfacial electron transfer, demonstrating that the interaction between the WSSe nanotube and molecules is weak. According to the above analysis, it can be determined that the adsorption of CO 2 by the pristine WSSe nanotube is physisorption.

The CO2 Adsorption on Defective Janus Wsse Nanotube
The pristine WSSe nanotube can be used as a gas collection system for physical CO2 adsorption. However, in order to convert the CO2 gas into value-added industrial raw materials through chemical reactions, chemical adsorption of CO2 is required, which requires the substrate to have a stronger adsorption capacity. Our earlier results have reported that introducing vacancy defects could effectively improve the stability of the geometric structures for some gas adsorption systems, making the adsorption capacity of the substrate increase [33][34][35].
Since the CO2 is more easily adsorbed on the Se side of WSSe nanotube, hereby, we applied the Se vacancy defects into the Janus WSSe nanotube to enhance its CO2 adsorption capacity, which also has been demonstrated to be more easily formed than the S and W vacancy defects in the WSSe layered material [33]. Based on the analysis on the elastic modulus, we find that a low Se vacancy concentration does not affect the mechanical property of the Janus WSSe nanotube drastically. (More details can be found in the Supporting Information and Figure S3). The calculated ads of CO2 molecule adsorbing on defective Janus WSSe nanotube is −1.41 eV, greatly exceeding the one (−0.19 eV) on the pristine Janus WSSe nanotube, indicating that the introduction of Se vacancy strengthens the CO2 adsorption. More interesting, as displayed in Figure 3a, the adsorbed CO2 molecule undergoes an obvious deformation from the initial linear shape into the bending one (∠OCO = 114.17°). Additionally, one of the C=O bonds in the adsorbed CO2 molecule (C-O2 bond) transforms into the C-O bond, and the C and O2 atoms bond to different W atoms, respectively. The obvious deformation demonstrates that the CO2 molecule could be activated by the defective Janus WSSe nanotube. However, the defective planar WSSe monolayer does not have such high activity. The adsorbed CO2 molecule on the defective planar WSSe monolayer keeps its linear shape (see Figure S4), and the adsorption energy in this case is only −0.20 eV. This phenomenon can be explained by the following reasons: (I) bending the planar structure allows more of the W atom area to be exposed, enlarging the contact surface of the CO2 molecule on the W atom; (II) the W atoms near the Se vacancy in the tubular structure WSSe have more electrons (0.15 e/atom) than the ones in the planar structure, according to the Bader charge results, which leads to easier electron transfer from W atoms on the WSSe nanotubes to the CO2 molecule and facilitates the formation of strong bonds.
In the following, we further discuss the enhanced adsorption of CO2 on WSSe nanotubes with the introduction of Se vacancies, from the aspects of CDD, electron

The CO 2 Adsorption on Defective Janus Wsse Nanotube
The pristine WSSe nanotube can be used as a gas collection system for physical CO 2 adsorption. However, in order to convert the CO 2 gas into value-added industrial raw materials through chemical reactions, chemical adsorption of CO 2 is required, which requires the substrate to have a stronger adsorption capacity. Our earlier results have reported that introducing vacancy defects could effectively improve the stability of the geometric structures for some gas adsorption systems, making the adsorption capacity of the substrate increase [33][34][35].
Since the CO 2 is more easily adsorbed on the Se side of WSSe nanotube, hereby, we applied the Se vacancy defects into the Janus WSSe nanotube to enhance its CO 2 adsorption capacity, which also has been demonstrated to be more easily formed than the S and W vacancy defects in the WSSe layered material [33]. Based on the analysis on the elastic modulus, we find that a low Se vacancy concentration does not affect the mechanical property of the Janus WSSe nanotube drastically. (More details can be found in the Supporting Information and Figure S3). The calculated E ads of CO 2 molecule adsorbing on defective Janus WSSe nanotube is −1.41 eV, greatly exceeding the one (−0.19 eV) on the pristine Janus WSSe nanotube, indicating that the introduction of Se vacancy strengthens the CO 2 adsorption. More interesting, as displayed in Figure 3a, the adsorbed CO 2 molecule undergoes an obvious deformation from the initial linear shape into the bending one (∠OCO = 114.17 • ). Additionally, one of the C=O bonds in the adsorbed CO 2 molecule (C-O 2 bond) transforms into the C-O bond, and the C and O2 atoms bond to different W atoms, respectively. The obvious deformation demonstrates that the CO 2 molecule could be activated by the defective Janus WSSe nanotube. However, the defective planar WSSe monolayer does not have such high activity. The adsorbed CO 2 molecule on the defective planar WSSe monolayer keeps its linear shape (see Figure S4), and the adsorption energy in this case is only −0.20 eV. This phenomenon can be explained by the following reasons: (I) bending the planar structure allows more of the W atom area to be exposed, enlarging the contact surface of the CO 2 molecule on the W atom; (II) the W atoms near the Se vacancy in the tubular structure WSSe have more electrons (0.15 e/atom) than the ones in the planar structure, according to the Bader charge results, which leads to easier electron transfer from W atoms on the WSSe nanotubes to the CO 2 molecule and facilitates the formation of strong bonds. transfer, and DOS. As mentioned before, after CO2 adsorption at the Se vacancy site, C and O2 atoms separately bond to W atoms. As plotted in Figure 3b, the electron transfer amount from the defective Janus WSSe nanotube to the CO2 molecules is up to 1.12 e. The formation of C-W and O-W bonds indicate that on the defective Janus WSSe nanotube, the CO2 adsorption is chemical adsorption. For the purpose of understanding the electronic origin of the chemisorption on the defective Janus WSSe nanotube, its corresponding DOS is calculated. For the defective Janus WSSe nanotube, its conduction band maximum (CBM) rises to a high level after the CO2 adsorption (see Figure 4a,b), which corresponds to the Bader charge result that the defective Janus WSSe nanotube loses 1.12 e. In addition, as shown in Figure 4c, there is an obvious orbital hybridization between the CO2 molecule and the defective Janus WSSe nanotube, which is mainly contributed by the O-p and C-p orbitals from the adsorbed molecule as well as the W-d orbitals from the W atoms in substrate bonding to the C and O2 atoms. This explains the phenomenon that the CO2 gas molecule is tightly attached to the defective Janus WSSe nanotube through the C-W and O-W bonds. In addition, the DOS of the CO2 molecules pre-and post-adsorption (see Figures 2a and S5) shows that an obvious delocalization of DOS occurs after adsorption, which means a severe electron redistribution in the adsorbed CO2 gas molecule, caused by the gained electrons from the substrate. The results above provide more evidence that the adsorption of CO2 by the defective WSSe nanotubes is chemisorption. In other words, the introduction of Se vacancy can well convert the physical adsorption of CO2 into chemical adsorption on the Janus WSSe nanotube.  In the following, we further discuss the enhanced adsorption of CO 2 on WSSe nanotubes with the introduction of Se vacancies, from the aspects of CDD, electron transfer, and DOS. As mentioned before, after CO 2 adsorption at the Se vacancy site, C and O2 atoms separately bond to W atoms. As plotted in Figure 3b, the electron transfer amount from the defective Janus WSSe nanotube to the CO 2 molecules is up to 1.12 e. The formation of C-W and O-W bonds indicate that on the defective Janus WSSe nanotube, the CO 2 adsorption is chemical adsorption.
For the purpose of understanding the electronic origin of the chemisorption on the defective Janus WSSe nanotube, its corresponding DOS is calculated. For the defective Janus WSSe nanotube, its conduction band maximum (CBM) rises to a high level after the CO 2 adsorption (see Figure 4a,b), which corresponds to the Bader charge result that the defective Janus WSSe nanotube loses 1.12 e. In addition, as shown in Figure 4c, there is an obvious orbital hybridization between the CO 2 molecule and the defective Janus WSSe nanotube, which is mainly contributed by the O-p and C-p orbitals from the adsorbed molecule as well as the W-d orbitals from the W atoms in substrate bonding to the C and O2 atoms. This explains the phenomenon that the CO 2 gas molecule is tightly attached to the defective Janus WSSe nanotube through the C-W and O-W bonds. In addition, the DOS of the CO 2 molecules pre-and post-adsorption (see Figures 2a and S5) shows that an obvious delocalization of DOS occurs after adsorption, which means a severe electron redistribution in the adsorbed CO 2 gas molecule, caused by the gained electrons from the substrate. The results above provide more evidence that the adsorption of CO 2 by the defective WSSe nanotubes is chemisorption. In other words, the introduction of Se vacancy can well convert the physical adsorption of CO 2 into chemical adsorption on the Janus WSSe nanotube.
Molecules 2023, 28, x FOR PEER REVIEW 5 of 12 transfer, and DOS. As mentioned before, after CO2 adsorption at the Se vacancy site, C and O2 atoms separately bond to W atoms. As plotted in Figure 3b, the electron transfer amount from the defective Janus WSSe nanotube to the CO2 molecules is up to 1.12 e. The formation of C-W and O-W bonds indicate that on the defective Janus WSSe nanotube, the CO2 adsorption is chemical adsorption. For the purpose of understanding the electronic origin of the chemisorption on the defective Janus WSSe nanotube, its corresponding DOS is calculated. For the defective Janus WSSe nanotube, its conduction band maximum (CBM) rises to a high level after the CO2 adsorption (see Figure 4a,b), which corresponds to the Bader charge result that the defective Janus WSSe nanotube loses 1.12 e. In addition, as shown in Figure 4c, there is an obvious orbital hybridization between the CO2 molecule and the defective Janus WSSe nanotube, which is mainly contributed by the O-p and C-p orbitals from the adsorbed molecule as well as the W-d orbitals from the W atoms in substrate bonding to the C and O2 atoms. This explains the phenomenon that the CO2 gas molecule is tightly attached to the defective Janus WSSe nanotube through the C-W and O-W bonds. In addition, the DOS of the CO2 molecules pre-and post-adsorption (see Figures 2a and S5) shows that an obvious delocalization of DOS occurs after adsorption, which means a severe electron redistribution in the adsorbed CO2 gas molecule, caused by the gained electrons from the substrate. The results above provide more evidence that the adsorption of CO2 by the defective WSSe nanotubes is chemisorption. In other words, the introduction of Se vacancy can well convert the physical adsorption of CO2 into chemical adsorption on the Janus WSSe nanotube.

Photocatalytic Performance of Defective WSSe Nanotube for CO 2 RR
The activation of the CO 2 gas molecule on the defective WSSe nanotube makes the further catalytic CO 2 reduction reaction possible. As displayed in Figure 5a, though the Se vacancy bring about some gap states, the defective Janus WSSe nanotube still keeps the semiconductor character with a narrower band gap of 0.83 eV (the band gap of the pristine Janus WSSe nanotube is 1.56 eV, see Figure 5b). In the following, we studied the photocatalytic performance of the defective Janus WSSe nanotube.

Photocatalytic Performance of Defective WSSe Nanotube for CO2RR
The activation of the CO2 gas molecule on the defective WSSe nanotube makes further catalytic CO2 reduction reaction possible. As displayed in Figure 5a, though th vacancy bring about some gap states, the defective Janus WSSe nanotube still keeps semiconductor character with a narrower band gap of 0.83 eV (the band gap of the p tine Janus WSSe nanotube is 1.56 eV, see Figure 5b). In the following, we studied photocatalytic performance of the defective Janus WSSe nanotube. In order to initiate the photocatalytic conversion of CO2, an efficient photocata must have a high photo-conversion efficiency. As shown in Figure 6a, there are sev significant light absorption peaks (over 10 5 cm −1 ) among the visible light area for pristine and defective Janus WSSe nanotubes, indicating they are promising cata candidates with visible-light responses. The highest absorption peak in the visible for the pristine and defective Janus WSSe nanotubes arrive 3.10 × 10 5 cm −1 (at 380.00 black line) and 2.96 × 10 5 cm −1 (at 380.00 nm, red line), which exceed the one of the pl Janus WSSe (1.30 × 10 5 cm −1 at 466.28 nm) [34] and are on par with some reported ph catalysts, namely, MoSSe/graphene (4.00 × 10 5 cm −1 at 500 nm) [36] and MoSSe/AlN ( × 10 5 cm −1 at 412 nm) [37]. Although the difference between the light absorption spe of the pristine and defective Janus WSSe nanotubes are not significant, as displaye Figure S6, in the infrared and visible regions, the optical absorption coefficient of defective Janus WSSe nanotube is higher than the one of the pristine Janus WSSe n tube, which is consistent with the fact that the defective Janus WSSe nanotube h smaller band gap than the pristine one. The non-zero absorbance value in the infr region (IR) of the defective WSSe nanotube ensures the utilization of IR photons. Th fore, the introduction of Se vacancy defects makes the Janus WSSe nanotube use pho in a relatively larger energy range. Additionally, the negligible difference of light sorption spectra between these two kinds of nanotubes may be caused by the fact the gap states are too weak in the defective Janus WSSe, where the concentration o vacancy is too low (just 4.17%). In the visible region, the reported optical absorption efficient of defective Janus WSSe monolayer with a higher concentration of Se vaca (6.25%) is more obviously higher than the pristine Janus WSSe monolayer [34], w agrees well with the results of nanotubes.
In order for a semiconductor to be active for photo-reduction of CO2, the band ed must be aligned with the potentials of the reduction half-reactions [38]. On top of tha band edge also needs to satisfy the oxidation potential of H2O/O2 because the oxy evolution reaction (OER) could consume the redundant photo-excited holes and pro the necessary H + + e − pair simultaneously. As shown in Figure 6b, the CBM in the p In order to initiate the photocatalytic conversion of CO 2 , an efficient photocatalyst must have a high photo-conversion efficiency. As shown in Figure 6a, there are several significant light absorption peaks (over 10 5 cm −1 ) among the visible light area for the pristine and defective Janus WSSe nanotubes, indicating they are promising catalyst candidates with visible-light responses. The highest absorption peak in the visible area for the pristine and defective Janus WSSe nanotubes arrive 3.10 × 10 5 cm −1 (at 380.00 nm, black line) and 2.96 × 10 5 cm −1 (at 380.00 nm, red line), which exceed the one of the planar Janus WSSe (1.30 × 10 5 cm −1 at 466.28 nm) [34] and are on par with some reported photocatalysts, namely, MoSSe/graphene (4.00 × 10 5 cm −1 at 500 nm) [36] and MoSSe/AlN (3.95 × 10 5 cm −1 at 412 nm) [37]. Although the difference between the light absorption spectra of the pristine and defective Janus WSSe nanotubes are not significant, as displayed in Figure S6, in the infrared and visible regions, the optical absorption coefficient of the defective Janus WSSe nanotube is higher than the one of the pristine Janus WSSe nanotube, which is consistent with the fact that the defective Janus WSSe nanotube has a smaller band gap than the pristine one. The non-zero absorbance value in the infrared region (IR) of the defective WSSe nanotube ensures the utilization of IR photons. Therefore, the introduction of Se vacancy defects makes the Janus WSSe nanotube use photons in a relatively larger energy range. Additionally, the negligible difference of light absorption spectra between these two kinds of nanotubes may be caused by the fact that the gap states are too weak in the defective Janus WSSe, where the concentration of Se vacancy is too low (just 4.17%). In the visible region, the reported optical absorption coefficient of defective Janus WSSe monolayer with a higher concentration of Se vacancy (6.25%) is more obviously higher than the pristine Janus WSSe monolayer [34], which agrees well with the results of nanotubes. redox capacity for both photocatalytic CO2RR and OER. Furthermore, our previous wo pointed out that [27] the dipole caused by the structural asymmetry introduces a built electric field with the direction from the Se layer to the S layer (see the pink arrow Figure 6b). In this case, the photoexcited electron and hole will run fast in opposite rections, causing high spatial separation of the electron-hole pairs, which surely su presses the recombination of photoexcited carriers. Next, we explore whether the reaction can be spontaneous under dynamic con tions. The case without any external potential (U = 0 V) is used to simulate the conditi in darkness. We first screen the favorable reaction path of CO2RR on the defective Jan WSSe nanotube (see Figure S7). The CO2RR-to-CH4 process involves eight p ton-coupled electron transfer steps (CO2 + 8H + + 8e − → 2H2O + CH4). The free energy agram and the corresponding intermediates for the CO2RR-to-CH4 are shown in Figu 7a. The most possible path is CO2 * → OCOH * → OCHOH * → OCH * → OCH2 * OCH3 * → O * →OH * → H2O*. The electrocatalytic steps, i.e., OCHOH * → OCH *, OCH → OCH2 *, OCH2 * → OCH3 *, and OCH3 * → O*, are exothermic by −0.41, −0.51, −0.15, a −1.43 eV, respectively; meanwhile, the other hydrogenation steps, i.e., CO2 * → OCOH OCOH * → OCHOH *, O * → OH *, and OH * → H2O *, are endothermic by 0.65, 0. 0.06, and 0.50 eV, respectively. The formation of OCOH * is the potential determini step (PDS) with a limiting potential (Ul) of −0.65 V. At the same time, we also inves gated the OER process on the S side of the defective Janus WSSe nanotube along the transfer pathway, i.e., H2O → OH * → OOH * → O2 (see Figure 7b) [18,27]. The free e ergy changes (ΔG) for the four different steps are endothermic by 1.81, 0.06, 1.75, and 1 eV, respectively. The formation of OH* is the PDS with a Ul of −1.81 V.
According to the free energy calculations mentioned above, it could be found th both the CO2RR and OER have endothermic steps; thus, they could not take place spo taneously without photo-irradiation. However, the high enough external potential su plied by the photo-excited carriers helps to overcome the Ul of these redox half-reactio making the redox half-reactions proceed spontaneously [39]. The extra potential of t photogenerated electrons/holes (Ue/Uh) is defined as the energy difference between H + / reduction potential and the CBM/VBM [18,[39][40][41]. According to our previous work [2 the Ue and Uh of the defective Janus WSSe nanotube at pH = 0 are 0.73 and 2.77 V, spectively, which are sufficient enough to separately cover the Ul of CO2RR and OE Therefore, in consideration of Ue and Uh, all the reduction and oxidation steps becom downhill (red dash lines in Figure 7a,b). That is to say, under the light irradiation, bo CO2RR and OER can operate spontaneously. In order for a semiconductor to be active for photo-reduction of CO 2 , the band edges must be aligned with the potentials of the reduction half-reactions [38]. On top of that, its band edge also needs to satisfy the oxidation potential of H 2 O/O 2 because the oxygen evolution reaction (OER) could consume the redundant photo-excited holes and provide the necessary H + + e − pair simultaneously. As shown in Figure 6b, the CBM in the photocatalytic redox capacity is above the CO 2 /CH 4 reduction potential, and the VBM is below the H 2 O/O 2 oxidation potential, indicating that the WSSe nanotubes have sufficient redox capacity for both photocatalytic CO 2 RR and OER. Furthermore, our previous work pointed out that [27] the dipole caused by the structural asymmetry introduces a built-in electric field with the direction from the Se layer to the S layer (see the pink arrow in Figure 6b). In this case, the photoexcited electron and hole will run fast in opposite directions, causing high spatial separation of the electron-hole pairs, which surely suppresses the recombination of photoexcited carriers.
Next, we explore whether the reaction can be spontaneous under dynamic conditions. The case without any external potential (U = 0 V) is used to simulate the condition in darkness. We first screen the favorable reaction path of CO 2 RR on the defective Janus WSSe nanotube (see Figure S7). The CO 2 RR-to-CH 4 process involves eight proton-coupled electron transfer steps (CO 2 + 8H + + 8e − → 2H 2 O + CH 4 ). The free energy diagram and the corresponding intermediates for the CO 2 RR-to-CH 4 are shown in Figure 7a. The most possible path is CO 2 * → OCOH * → OCHOH * → OCH * → OCH 2 * → OCH 3 * → O * →OH * → H 2 O *. The electrocatalytic steps, i.e., OCHOH * → OCH *, OCH * → OCH 2 *, OCH 2 * → OCH 3 *, and OCH 3 * → O *, are exothermic by −0.41, −0.51, −0.15, and −1.43 eV, respectively; meanwhile, the other hydrogenation steps, i.e., CO 2 * → OCOH *, OCOH * → OCHOH *, O * → OH *, and OH * → H 2 O *, are endothermic by 0.65, 0.15, 0.06, and 0.50 eV, respectively. The formation of OCOH * is the potential determining step (PDS) with a limiting potential (U l ) of −0.65 V. At the same time, we also investigated the OER process on the S side of the defective Janus WSSe nanotube along the 4 e transfer pathway, i.e., H 2 O → OH * → OOH * → O 2 (see Figure 7b) [18,27]. The free energy changes (∆G) for the four different steps are endothermic by 1.81, 0.06, 1.75, and 1.30 eV, respectively. The formation of OH * is the PDS with a U l of −1.81 V. Usually, the hydrogen evolution reaction (HER) is considered to be an important competitive side reaction in the catalytic CO2RR [42,43]. Next, we investigated the competitive relationship between CO2RR and HER in the defective Janus WSSe nanotubes. Based on the Brønsted-Evans-Polanyi relation [44,45], the reaction with lower Gibbs, ΔG, values has a smaller reaction barrier; thus, it is more favorable for kinetics. Accordingly, the ΔG for H * formation energy (ΔGH *) is calculated ( Figure 8a) and compared with the one for CO2 * formation energy (ΔGCO2 *). As shown in Figure 8b, ΔGCO2 * (−0.67 eV) is more negative than ΔGH* (−0.15 eV), which ensures that the active sites are preferred to be occupied by CO2 *. Therefore, the defective Janus WSSe nanotube is more selective for CO2RR over HER.

Computational Methods
In our work, all the computational models are constructed with the DeviceStudio software [46]. In addition, the Geometric relaxation and electronic structure were conducted based on DFT simulations employing DS-PAW software [47]. The exchange-correlation energy of Perdew-Burke-Ernzerhof (PBE) was employed [48]. To depict the van der Waals (vdW) coupling in the adsorption system, we used the zero-damping DFT-D3 method suggested from Grimme [49]. All internal coordinates with fixed lattice constants were permitted to relax during the optimization process. The sampling integration of the Brillouin zone was performed in accordance with the Monkhorst-Pack scheme [50], and the structure optimization and electronic properties are calculated with a 1 × 1 × 4 K-point. The value of 500 eV was chosen as the cutoff energy of plane-wave basis. We set the periodic boundary condition along the z-axis and put more than 10.8 Å vacuum spaces along the x and y directions to avoid the interaction between adjacent nanotubes. Periodic boundary conditions were set on the z-axis and a vacuum space of more than 10.8 Å was applied on the x-and y-axes to evade adjacent According to the free energy calculations mentioned above, it could be found that both the CO 2 RR and OER have endothermic steps; thus, they could not take place spontaneously without photo-irradiation. However, the high enough external potential supplied by the photo-excited carriers helps to overcome the U l of these redox half-reactions, making the redox half-reactions proceed spontaneously [39]. The extra potential of the photogenerated electrons/holes (U e /U h ) is defined as the energy difference between H + /H 2 reduction potential and the CBM/VBM [18,[39][40][41]. According to our previous work [27], the U e and U h of the defective Janus WSSe nanotube at pH = 0 are 0.73 and 2.77 V, respectively, which are sufficient enough to separately cover the U l of CO 2 RR and OER. Therefore, in consideration of U e and U h , all the reduction and oxidation steps become downhill (red dash lines in Figure 7a,b). That is to say, under the light irradiation, both CO 2 RR and OER can operate spontaneously.
Usually, the hydrogen evolution reaction (HER) is considered to be an important competitive side reaction in the catalytic CO 2 RR [42,43]. Next, we investigated the competitive relationship between CO 2 RR and HER in the defective Janus WSSe nanotubes. Based on the Brønsted-Evans-Polanyi relation [44,45], the reaction with lower Gibbs, ∆G, values has a smaller reaction barrier; thus, it is more favorable for kinetics. Accordingly, the ∆G for H * formation energy (∆G H* ) is calculated (Figure 8a) and compared with the one for CO 2 * formation energy (∆G CO2* ). As shown in Figure 8b, ∆G CO2* (−0.67 eV) is more negative than ∆G H* (−0.15 eV), which ensures that the active sites are preferred to be occupied by CO 2 *. Therefore, the defective Janus WSSe nanotube is more selective for CO 2 RR over HER. Usually, the hydrogen evolution reaction (HER) is considered to be an important competitive side reaction in the catalytic CO2RR [42,43]. Next, we investigated the competitive relationship between CO2RR and HER in the defective Janus WSSe nanotubes Based on the Brønsted-Evans-Polanyi relation [44,45], the reaction with lower Gibbs, ΔG values has a smaller reaction barrier; thus, it is more favorable for kinetics. Accordingly the ΔG for H * formation energy (ΔGH *) is calculated (Figure 8a) and compared with the one for CO2 * formation energy (ΔGCO2 *). As shown in Figure 8b, ΔGCO2 * (−0.67 eV) is more negative than ΔGH* (−0.15 eV), which ensures that the active sites are preferred to be occupied by CO2 *. Therefore, the defective Janus WSSe nanotube is more selective for CO2RR over HER.

Computational Methods
In our work, all the computational models are constructed with the DeviceStudio software [46]. In addition, the Geometric relaxation and electronic structure were conducted based on DFT simulations employing DS-PAW software [47]. The exchange-correlation energy of Perdew-Burke-Ernzerhof (PBE) was employed [48]. To depict the van der Waals (vdW) coupling in the adsorption system, we used the ze-

Computational Methods
In our work, all the computational models are constructed with the DeviceStudio software [46]. In addition, the Geometric relaxation and electronic structure were conducted based on DFT simulations employing DS-PAW software [47]. The exchange-correlation energy of Perdew-Burke-Ernzerhof (PBE) was employed [48]. To depict the van der Waals (vdW) coupling in the adsorption system, we used the zero-damping DFT-D3 method suggested from Grimme [49]. All internal coordinates with fixed lattice constants were permitted to relax during the optimization process. The sampling integration of the Brillouin zone was performed in accordance with the Monkhorst-Pack scheme [50], and the structure optimization and electronic properties are calculated with a 1 × 1 × 4 K-point. The value of 500 eV was chosen as the cutoff energy of plane-wave basis. We set the periodic boundary condition along the z-axis and put more than 10.8 Å vacuum spaces along the x and y directions to avoid the interaction between adjacent nanotubes. Periodic boundary conditions were set on the z-axis and a vacuum space of more than 10.8 Å was applied on the x-and y-axes to evade adjacent nanotubes from interacting with each other. Moreover, the ∆G of CO 2 RR and OER were calculated using the computational hydrogen electrode (CHE) model [51]. Additional details of the Gibbs free energy simulations are available in the Supporting Information.
The E ads of the CO 2 on the WSSe nanotube was obtained from the following equation [52,53], where E total was the total energy of the adsorption system, while E CO 2 and E sub separately were the total energies of the isolated CO 2 molecule and the clean Janus WSSe nanotube. A higher negative E ads indicated a more favorable exothermic adsorption. The plane-integrated CDD was carried out in accordance with the equation, where ρ total , ρ CO 2 , and ρ sub separately were the charge density of the adsorption system, adsorbed CO 2 molecule, and substrate. The absorption coefficient, a(ω), used to estimate the solar energy gathering capacity was given by the following equation [27], where ε 1 and ε 2 frequently were the real and imaginary parts of the frequency-dependent dielectric function, while c was the speed of light under vacuum.

Conclusions
In this paper, based on the first-principles calculations, we investigate the performance of the defective Janus WSSe nanotube for the photocatalytic CO 2 RR. The introduction of Se vacancy could significantly increase the amount of interfacial transferred electrons and lead to obvious electron orbital hybridization between adsorbates and substrates, making the CO 2 adsorption on the Janus WSSe nanotube transform into chemisorption from physisorption. Strong chemisorption enables defective Janus WSSe nanotubes to be highly active and selective against CO 2 RR. In addition, the extra potential from photoproduced carriers is high enough to trigger spontaneous CO 2 RR and OER simultaneously on the defective Janus WSSe nanotube. For the first time, our work theoretically predicts the high photocatalytic performance of the defective Janus WSSe nanotube on CO 2 RR, which promisingly will stimulate extensive interests from material science and chemistry communities to realize our vision.

Supplementary Materials:
The following Supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/molecules28124602/s1, Figure S1: Diameter, W-S bond length, W-S bond length and Se-S height of the pristine Janus WSSe nanotube; Figure S2: The relative adsorption energy and the location for the CO 2 gas molecule passing through the pristine WSSe nanotube wall; Figure S3: Relative value of total energy variations as well as their corresponding fittings for the pristine and defective Janus WSSe nanotubes with respect to Strain ε along the tube axis; Figure S4: Top view and Side view of CO 2 gas molecules adsorbed at the Se vacancy of Janus WSSe monolayer; Figure S5: The enlarged view for the partial density of States of CO 2 portion from the adsorption System; Figure S6: The enlarged view the optical absorbance of pristine and defective Janus WSSe nanotubes at wavelength of 500-900 nm; Figure S7: The Search process for the minimum energy reaction pathways of the CO 2 reduction reactions on defective Janus WSSe nanotube; Table S1: The amount of charge transfer for C and O atoms of CO 2 gas molecules adsorbed in pristine and defective Janus WSSe nanotubes, respectively; Free energy difference in the CO2RR and OER; Table S2: Zero-pint energy correction and entropy contribution of molecules and adsorbates in this Study. Refs. [51,54]